As a veteran of the Lieberman wars, when official Washington acted as if primarying a "respected" sitting senator was the equivalent to setting up the guillotines, I've been amused at the inconsistent application of that view of primarying incumbents since.
It comes down to:
Primarying centrists, hawks, reliable corrupt torchbearers of the Washington Consensus: unfathomable, unforgivable violence
Primarying left-coded candidates, even if they aren't all that lefty: good, smart.